Engines Go Back to the Future

By CHERYL JENSEN

Published: December 4, 2005

GERMAN automakers gave up pushrod engines long ago, in favor of more complex overhead-cam power plants. The Japanese have essentially quit making the old-design engines. Ford is down to just a couple.

But Chrysler, with seven, and especially General Motors, with about a dozen in 21 different forms, remain bastions of the pushrod engine, also known as an overhead-valve design.

Not only has G.M. continued to carry forward older engine designs, like the famous ''small block V-8'' in your grandfather's 1955 Bel Air (and your son's 2005 Corvette), it has been designing new ones. The Chevy Impala offers two pushrod V-6's that are new for 2006.

G.M. says the new engines share almost nothing with the old ones. The Impala's base engine, a 3.5-liter V-6, was actually developed from the more powerful 3.9 V-6; the two share more than 80 percent of their parts.

While the Impala's powertrains may seem caught in a piston-pushing time warp, the new-old engines were designed with innovations like variable valve timing, which provides a broader power range and produces lower emissions.

Further, G.M. and Chrysler say that pushrod engines lend themselves better than overhead-cam engines -- the two companies make plenty of those, too -- to another bit of fuel-saving technology that is becoming popular: deactivation of half the cylinders at cruising speed.

But money is a big factor in G.M.'s back-to-the-future powertrains. Brett Smith, director of product and technology forecasting for the Center for Automotive Research in Ann Arbor, Mich., said the company saved an estimated $800 a vehicle by sticking with pushrod motors, which cost less to make largely because they contain fewer parts. Dollars saved on the engines can be used to add other features to the cars.

Some enthusiasts defend pushrod engines, which usually have two valves per cylinder (rather than the three or more valves common among overhead-cam designs) for their strong low-end thrust.

A potentially bigger deficiency of the new Impala is its four-speed automatic transmission; competitors have five or six speeds. Additional gears can improve acceleration and fuel economy. G.M. is working on six-speed automatics for front- and all-wheel-drive cars, but they were not available in time for the 2006 Impala.